This invention relates generally to methods for the synthesis of stable, aqueous, mainly acidic solutions of hydrogen peroxide and improved electrocatalytic electrodes, cells and systems for more efficient production of high concentration peroxide solutions.
Hydrogen peroxide is a strong oxidant, which is also an environmentally favored chemical for various applications, such as in chemical synthesis, water treatment, pulp and paper bleaching and waste treatment. Hydrogen peroxide is also widely used as an alternative for chlorine in view of its more benign affects on the environment. Use of hydrogen peroxide by various industries is such that the demand has been growing at a steady rate ( greater than 7% annually). Consequently, new and improved processes for commercial and on-site production of hydrogen peroxide would be of substantial interest to many industries.
Hydrogen peroxide has been produced by chemical methods, as well as electrochemical methods. Representative catalytic chemical methods include such processes as oxidation of secondary alcohols, e.g. isopropanol, to ketone and peroxide; anthraquinone auto-oxidation by cyclic reduction/oxidation; oxidation of alkali metals to peroxides; metal to peroxide followed by hydrolysis; synthesis by the direct combination of hydrogen and oxygen over noble metal catalyst, and so on.
The above mentioned cyclic anthraquinone process is an auto-oxidation process, often chosen for large scale manufacturing of hydrogen peroxide. This chemical process relies on mixed organic solvents to maximize the solubility of anthraquinone to achieve high yields of peroxide. In this process, an alkylanthraquinone, a quinoid, is chemically reduced with hydrogen in the presence of a catalyst to the corresponding 2-alkyl 9,10-dihydroanthraquinone. The 2-alkyl 9,10-dihydroanthraquinone is then oxidized in the presence of oxygen back to the corresponding quinoid, plus hydrogen peroxide. Desirably, the alkylanthraquinone is then available for separation and recovery as recycle in the further synthesis of hydrogen peroxide.
Some of the more significant problems associated with the cyclic anthraquinone process relate to peroxide contamination from the organic solvent system and the requirement of catalyst removal prior to the oxidation step as a safety precaution to avoid potentially explosive reactions. Consequently, several separation steps are required in the process. While a number of improvements have been made in the cyclic anthraquinone process, a purely aqueous process would be highly desirable for ease of operation.
Other significant problems associated with catalytic chemical methods have included potentially explosive reactions occurring, i.e., safety concerns with hydrogen and oxygen reacting; the generation of undesirable by-products, like acetone, and so on.
Generally, electrochemical methods for the synthesis of hydrogen peroxide offer some important advantages over chemical methods, including higher purity, fewer separation steps, fewer unwanted by-products, greater safety and fewer environmental concerns.
Representative methods for the electrochemical production of hydrogen peroxide include, for example, the electrolysis of ammonium sulfate followed by the hydrolysis of persulfate to peroxide; cathodic reduction of oxygen to alkaline peroxide solution; redox polymer with quinoid groups in the reduced form to effectuate oxygen reduction to peroxide; electrochemical reduction of a quinone to continuously regenerate hydroquinone in aqueous medium, etc.
Notwithstanding the number of substantial advantages associated with electrochemical synthesis methods for the production of hydrogen peroxide, some electrolytic processes have experienced shortcomings. For example, in the electrolytic method wherein sulfuric acid or sulfate salts of potassium, sodium or ammonium are oxidized to persulfate, capital equipment costs can be high due to costly platinum anodes corroding resulting in losses in valuable platinum metal.
The so called Dow Process for on-site electrolytic production of hydrogen peroxide employs cathodic reduction of oxygen in a trickle bed cell. The hydrogen peroxide solution is used directly for pulp bleaching, de-inking recycled paper, etc. However, the hydrogen peroxide produced according to this method has an alkaline pH, rendering it less stable. Consequently, alkaline pH peroxides produced by such methods are not entirely suitable for transportation, long-term storage, or for use in other applications, like mining, chemical synthesis, and certain environmental applications.
Formation of hydrogen peroxide through electrochemical oxygen reduction is described in several patents and technical publications, including U.S. Pat. No. 5,112,702 to Berzins et al; Canadian Pat. 2,103,387 to Drackett; German DE 4 311 665 to Hilricha, et al; Japan. Kokai JP 06,600,389 (1994) to Otsuka, et al; Electrochem acta, 35(2), 1990, 319-22. Kalu, et al, J. Applied Electrochemistry, 20, p. 932-940, 1990, describe a method for simultaneous production of sodium chlorate and hydrogen peroxide using a cathodic oxygen reduction cell. Foller et al, J. Applied Electrochem., 25, p. 613-27, 1995, reported on the use of gas diffusion electrodes for the preparation of hydrogen peroxide. However, the peroxides had an alkaline pH, and were of generally poor stability.
While the chemical and electrochemical processes for the production of hydrogen peroxide each offer several important benefits, it would be highly desirable to have a modified process which offers the advantages of an aqueous medium for safety and environmental concerns, along with higher purity and minimal by-products of an electrochemical process, but which mimics certain features of established catalytic chemical process technology to enable the production of more stable peroxides at acid pH ranges, and at higher yields.
The literature describes methods relating to the preparation and use of redox polymers for the chemical and electrochemical synthesis of hydrogen peroxide. For example, Manecke, Angew. Chem., 68, 582, 1956, used a redox polymer formed by the condensation polymerization of hydroquinone and formaldehyde. In a column reactor, workers were able to produce 2N hydrogen peroxide (3% by wt.) by passing oxygen-saturated water and recycling through the bed. Manecke, et al, Electrochem., 62, 311, 1958; Izoret, G., Ann. Chim., (Paris), 7, 151, 1962 describe passing a solution of sodium dithionite or other reducing agent for regenerating a resin. U.S. Pat. No. 2,992,899 (Manecke) disclose a process for peroxide synthesis using an insoluble oxidation/reduction resin by passing oxygen saturated water through a bed of the resin.
U.S. Pat. No. 4,647,359 to Lindstrom discloses the preparation of electrocatalytic gas diffusion electrodes employing noble metal catalyzed carbon cloth.
U.S. Pat. No. 6,274,114 to Ledon et al disclose a two step electrochemical process for the generation of hydrogen peroxide wherein cobalt is oxidized in an electrochemical cell to form a Co+3 complex, which is then reacted with oxygen to form peroxide and reduced cobalt (Co+2).
In a 1969 patent, U.S. Pat. No. 3,454,477 to Grangaard, there is disclosed an electrochemical method for producing hydrogen peroxide using a quinone redox polymer deposited on a porous graphite cathode, and operated in an alkaline electrolyte. However, the performance of this system indicates poor stability of the electrode, as well as the production of peroxides at low concentrations generated at current efficiencies of less than 25%.
Accordingly, there is a need for more reliable and efficient semi-electrochemical/chemical or hybrid methods, apparatus and systems for the synthesis of hydrogen peroxide in stable aqueous medium at high concentrations, and which are suitable for scaling-up for large manufacturing installations, as well as for smaller on-site peroxide generation.
It is therefore one principal object of this invention to provide for improved methods for the synthesis of aqueous solutions of hydrogen peroxide which allow for the omission of organic solvents. The more environmentally acceptable and safer semi-electrochemical/chemical process enables the preparation of more stable, concentrated solutions of peroxides mostly in neutral to acid aqueous mediums, i.e., at pHs of 7 or less, and more preferably at pH ranges from about 0 to about 6 or less.
The methods of the present invention also provide for more efficient reduction of oxygen to peroxide than achieved heretofore, typically providing solutions of peroxides of about 1M or greater, at cathode current efficiences of at least 35 percent, and more preferably, at efficiencies of at least 50 percent to as high as 95 percent, or more. The higher current efficiencies also translate into reduced power consumption.
Generally, the improved methods for the synthesis of hydrogen peroxide provide for:
passing a current through an aqueous electrolyte solution between an anode and cathode. The pH of the electrolyte solution prepared with a mineral acid or acid salt, preferably ranges from about 0 to about  less than 7. The cathode comprises at least a redox catalyst and a conductive substrate therefor. The redox catalyst is one that possesses both oxidation and reduction states, and may be comprised of any suitable catalyst which is capable of reacting with oxygen when in a reduced (reduction) state to form hydrogen peroxide at current efficiencies of at least 35 percent, and is also capable of reduction electrochemically when in an oxidized (oxidation) state.
The method includes substantially simultaneously oxidizing water at the anode of an electrolytic cell to form oxygen and protons, which in-turn are transported to the cathode, preferably in the environment of a compartmentalized electrolytic cell to prevent destruction of peroxide at the counter electrode or anode. Simultaneously, the redox catalyst of the cathode, in the presence of the protons from the anodic reaction of water, is continuously reduced electrochemically, i.e., by cathodic reduction. The reduced electrocatalyst in-turn reacts with a source of oxygen, such as air or an oxygen supply introduced at the cathode to generate hydrogen peroxide, preferably at current densities of at least 50 mA/cm2. The redox catalyst (bound to the cathode), which becomes oxidized in the process, is continuously regenerated electrochemically, i.e. cathodic reduction, for recycling and for further use in the process, and so on.
While syntheses are performed at cathode current efficiencies of at least 35 percent, the improved methods of the invention more often have been found to generate hydrogen peroxide solutions at more stable acid pHs at efficiencies from at least about 50 percent to about 99 percent, and at molar concentrations from about 1.0M to about 2.0M, and greater (about 3% or more). Because of the greater stability of the peroxide solution in the acidic pH range, the solutions can be concentrated further, if necessary, by distillation methods without degradation occurring.
While the foregoing reactions have been described as being performed in multiple steps, e.g., reactions at the anode, and at the cathode for purposes of clarity, it is to be understood the multiplicity of reactions taking place occur substantially simultaneously, and essentially in a single step.
It will be observed from the foregoing summary, the objectives of providing a method for the synthesis of solutions of peroxide without contamination from organic solvents, and further, without the usual multiple separation steps for avoiding potentially hazardous reactions from occurring, are achieved with the semi-electrochemical/chemical methods of the present invention.
It is yet a further object of the invention to provide novel catalytic electrodes, namely gas diffusion electrodes and membrane electrode assemblies (or solid polymer electrolytes) comprising the redox catalysts as described hereinabove, and electrolytic cells equipped therewith, including systems for large scale manufacturing and smaller on-site generators of hydrogen peroxide solutions according to methods herein described.
Also included within the objectives of this invention, are methods for the fabrication of the improved cathodes of the invention with suitable redox catalysts, such as quinone monomers or polymers including substituted and unsubstituted benzoquinone, anthraquinone, napthaquinone, and mixtures thereof. Other useful redox catalysts of the invention include azo compounds having the xe2x80x94Nxe2x95x90Nxe2x80x94 structure, which have been found to provide for stable reduction reactions occurring over a broader pH range of 14 or less.